tracing: Fix a possible race when disabling buffered events

Function trace_buffered_event_disable() is responsible for freeing pages
backing buffered events and this process can run concurrently with
trace_event_buffer_lock_reserve().

The following race is currently possible:

* Function trace_buffered_event_disable() is called on CPU 0. It
  increments trace_buffered_event_cnt on each CPU and waits via
  synchronize_rcu() for each user of trace_buffered_event to complete.

* After synchronize_rcu() is finished, function
  trace_buffered_event_disable() has the exclusive access to
  trace_buffered_event. All counters trace_buffered_event_cnt are at 1
  and all pointers trace_buffered_event are still valid.

* At this point, on a different CPU 1, the execution reaches
  trace_event_buffer_lock_reserve(). The function calls
  preempt_disable_notrace() and only now enters an RCU read-side
  critical section. The function proceeds and reads a still valid
  pointer from trace_buffered_event[CPU1] into the local variable
  "entry". However, it doesn't yet read trace_buffered_event_cnt[CPU1]
  which happens later.

* Function trace_buffered_event_disable() continues. It frees
  trace_buffered_event[CPU1] and decrements
  trace_buffered_event_cnt[CPU1] back to 0.

* Function trace_event_buffer_lock_reserve() continues. It reads and
  increments trace_buffered_event_cnt[CPU1] from 0 to 1. This makes it
  believe that it can use the "entry" that it already obtained but the
  pointer is now invalid and any access results in a use-after-free.

Fix the problem by making a second synchronize_rcu() call after all
trace_buffered_event values are set to NULL. This waits on all potential
users in trace_event_buffer_lock_reserve() that still read a previous
pointer from trace_buffered_event.

Link: https://lore.kernel.org/all/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]

Cc: [email protected]
Fixes: 0fc1b09 ("tracing: Use temp buffer when filtering events")
Signed-off-by: Petr Pavlu <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: petrpavlu

kernel/trace/trace.c

@@ -2791,13 +2791,17 @@ void trace_buffered_event_disable(void)
 		free_page((unsigned long)per_cpu(trace_buffered_event, cpu));
 		per_cpu(trace_buffered_event, cpu) = NULL;
 	}
+
 	/*
-	 * Make sure trace_buffered_event is NULL before clearing
-	 * trace_buffered_event_cnt.
+	 * Wait for all CPUs that potentially started checking if they can use
+	 * their event buffer only after the previous synchronize_rcu() call and
+	 * they still read a valid pointer from trace_buffered_event. It must be
+	 * ensured they don't see cleared trace_buffered_event_cnt else they
+	 * could wrongly decide to use the pointed-to buffer which is now freed.
 	 */
-	smp_wmb();
+	synchronize_rcu();
 
-	/* Do the work on each cpu */
+	/* For each CPU, relinquish the buffer */
 	on_each_cpu_mask(tracing_buffer_mask, enable_trace_buffered_event, NULL,
 			 true);
 }